Looks good to me. Only thing worth pointing out that I see is that assertWarns's def line will fit within 79 chars now.

Also, assertNotIsInstance's docstring is wrong. You're not changing that, so I guess that deserves another ticket if you don't want to fix it while you're here, or if you'd prefer that anyhow. The reason I mention it is that now that the assertion methods are defined using positive language, perhaps all of the docstrings might be clearer if they used that too so that you don't need any mental gymnastics to read them. That can go in another ticket too I guess if that's something reasonable sounding.